Organic EL elements have been keenly studied and developed (see, Patent Documents 1 and 2 and Non-Patent Document 1). In a fundamental structure of the organic EL element, a layer containing a light-emitting organic compound (hereinafter also referred to as light-emitting layer) is interposed between a pair of electrodes. The organic EL element has attracted attentions as a next-generation flat panel display element owing to characteristics such as feasibility of being thinner and lighter, high speed response to input signals, and capability of direct current low voltage driving.
In addition, a display using such a light-emitting element has a feature that it is excellent in contrast and image quality, and has a wide viewing angle. Further, since an organic EL element is a plane light source, it is considered that the light-emitting element is applied as a light source such as a backlight of a liquid crystal display and a lighting device.
The light-emission mechanism of organic EL elements is a carrier-injection system. That is, by application of voltage with a light-emitting layer interposed between electrodes, electrons and holes injected from the electrodes are recombined to make a light-emitting substance excited, and light emitted when the excited state returns to a ground state. There are two types of the excited states which are possible: a singlet excited state (S*) and a triplet excited state (T*). In addition, the statistical generation ratio thereof in a light-emitting element is considered to be S*:T*=1:3.
In general, the ground state of a light-emitting organic compound is a singlet state. Thus, light emission from the singlet excited state (S*) is referred to as fluorescence because it is caused by electron transition between the same spin multiplicities. On the other hand, light emission from a triplet excited state (T*) is referred to as phosphorescence where electron transition occurs between different spin multiplicities. Here, in a compound emitting fluorescence (hereinafter referred to as fluorescent compound), in general, phosphorescence is not observed at room temperature, and only fluorescence is observed. Accordingly, the internal quantum efficiency (the ratio of generated photons to injected carriers) in a light-emitting element using a fluorescent compound is assumed to have a theoretical limit of 25% based on S*:T*=1:3.
On the other hand, when a compound emitting phosphorescence (hereinafter referred to as phosphorescent compound) is used, an internal quantum efficiency of 100% can be theoretically achieved. In other words, higher emission efficiency can be obtained than that when a fluorescent compound is used. Therefore, the light-emitting element using a phosphorescent compound has been actively developed in recent years in order to achieve a highly efficient light-emitting element. As the phosphorescent compound, in particular, an organometallic complex that has iridium or the like as a central metal has particularly attracted attentions because of its high phosphorescence quantum yield; for example, an organometallic complex that has iridium as a central metal is disclosed as a phosphorescent material in Patent Document 1.
When a light-emitting layer of a light-emitting element is formed using a phosphorescent compound described above, in order to inhibit concentration quenching or quenching due to triplet-triplet annihilation in the phosphorescent compound, the light-emitting layer is often formed such that the phosphorescent compound is dispersed in a matrix of another compound. Here, the compound forming the matrix is called host, and the compound dispersed in the matrix, such as a phosphorescent compound, is called guest (or a dopant).
In addition, Patent Document 2 or Non-Patent Document 1 describes a method for forming a light-emitting layer with use of a mixture of a material with a high electron-transport property and a material with a high hole-transport property. For example, in Non-Patent Document 1, a light-emitting device is proposed, in which two materials of a tris(8-quinolinolato)aluminum complex (abbreviation: Alq3) with a high electron-transport property and 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB) with a high hole-transport property and methylquinacridon (abbreviation: mqa) as a guest (dopant) are used.
In the light-emitting layer having such a structure, two kinds of hosts have separate functions, so that electrons and holes can be transferred with good balance. In other words, electrons are transferred by Alq3 and holes are transferred by NPB and the electrons and holes reach mqa, so that mqa can be excited. Only fluorescence is emitted from mqa, but in Patent Document 2 discloses that light emission from a triplet excited state can be obtained by using a phosphorescent compound as a guest.